The present invention relates generally to a static dissipative textile having an electrically conductive surface which is achieved by coating the textile with an electrically conductive coating in a wide variety of patterns. The electrically conductive coating is generally comprised of a conducting agent and a binding agent, and optionally a dispersing agent and/or a thickening agent. The static dissipative textile generally comprises a fabric which may be screen printed or otherwise coated with an electrically conductive coating on the backside of the fabric so that the conductive coating does not interfere with the appearance of the face of the fabric. The economically produced fabric exhibits relatively permanent static dissipation properties and conducts electric charge at virtually any humidity, while the conductive coating does not detrimentally affect the overall appearance or tactile properties of the fabric. The fabric may be ideal for use in such end-use products as automotive upholstery; commercial and residential upholstery; cleanroom garments, wipes, and mops; napery; and apparel. Also encompassed within this invention is a method for producing a static dissipative textile having an electrically conductive surface.
It is generally known that some textile fabrics have inherent static generation problems, particularly synthetic fabrics such as polyester or nylon, and particularly in low humidity environments. Static generation occurs typically when two objects are brought into contact with each other and then separated. Generation is usually exacerbated when the objects are rubbed against each other, such as, for example, when two fabrics are rubbed against each other, resulting in a charge transfer between the two objects. This resulting potential difference between the object incurring the charge transfer and the surrounding environment, which may be tens of thousands of volts, can lead to uncomfortable and dangerous electric shocks. Static shocks can also result in damage to sensitive electronic components such as computer chips and sensors manufactured, for example, in cleanroom environments. Since one of the objects may be a fabric, the need exists for static dissipative fabrics to eliminate or reduce static electric charge created when an object is separated from a fabric.
One method of reducing the static charge on a fabric is to treat the fabric with a topical anti-static agent. These agents, which are commercially available, are typically quaternary ammonium salts, or ionic solutions containing small ions such as lithium ions. This type of anti-static treatment for fabric is disclosed, for example, in U.S. Pat. No. 5,643,865 to Mermelstein et al., U.S. Pat. No. 5,622,925 to de Buzzaccarini et al., and U.S. Pat. No. 5,254,268 to Schwartz. Other topical agents reduce static shock by lubricating the surface of the fabric with a hand modifier or softening agent thereby decreasing the friction between the fabric and the object rubbed against it. Both of these approaches suffer from a lack of durability to repeating abrasion. The softening agents and conducting finishes are partially removed during each abrasion or rubbing event; thus, the treatment is not permanent. Also, the fabric may develop issues with crocking. In addition, the conducting mechanism of the topical treatments depends on the presence of a small amount of water. Therefore, their effectiveness is quite limited in low humidity environments such as those encountered during winter months.
A method of producing a relatively permanent anti-static fabric that performs at substantially all humidity levels is to provide electrical conductivity to the fabric by the incorporation of conductive yarns into the fabric during the fabric formation process. The number and frequency of the conductive yarns, as well as their proximity to the surface of the fabric generally determine the amount of conductivity, and ultimately the amount of static protection provided by a particular fabric. In order to increase the effectiveness of static dissipation, the conducting yarns should intersect each other, thus forming a conductive grid. This method is used in many end-use applications such as in cleanroom garments and anti-static wipes. This method is disclosed, for example, in U.S. Pat. Nos. 4,557,968 and 4,606,968 both to Thornton et al. However, this method is limited by the high cost of conductive yarns and the cost of weaving, knitting, or stitching fabrics with these conductive yarns. Additionally, these conductive yarns are usually colored such that they may be undesirably visible on the face of the fabric. Furthermore, an end-use determination for a fabric must be made in advance of the fabric formation process so that the conductive yarns may be incorporated into the fabric at the onset of fabric formation.
Still another method of producing anti-static fabrics is to treat the entire surface of the fabric with a conductive paste or coating. This coating can be in the form of an intrinsically conducting polymer, such as that disclosed in U.S. Pat. No. 4,803,096 to Kuhn et al., or in the form of a conductive particle dispersed in a non-conducting matrix such as that described in U.S. Pat. No. 5,804,291 to Fraser. Although these methods overcome the limitations of topical treatments and are generally less expensive than incorporation of conductive yarns, they suffer from the fact that conductive coatings are normally highly colored and are often visible on the face of the fabric when used over the entire surface of the fabric. Also, the hand (or feel), drape, and air porosity of the fabric can be influenced adversely by impregnating the entire surface of the fabric with a matrix containing conductive particles.
Other methods have been disclosed in which an entire substrate is coated with a conductive polymer and then selected portions of the conductive polymer are removed. For example, U.S. Pat. No. 5,624,736 to DeAngelis et al. teaches a method in which a substrate is coated with a conductive polymer across its entire surface. The fabric is then coated in select areas with a protective film. The substrate is then subjected to a third treatment in which a chemical etching agent is used to remove the conductive polymer from the exposed portions of the substrate which were not covered with the protective film. Finally, the substrate is rinsed to remove the excess etching agent. Such a process, with so many operational steps, is rather complicated and lengthy and, like any process which involves coating an entire substrate only to remove large portions of the coating, necessarily involves a good deal of material loss. Also, this method leaves an insulating coating over the conducting areas, thus reducing the effectiveness of static dissipation. This method further suffers from the lack of breathability imparted to the conductive areas by the protective film. Another example of patterned conducting textile materials is disclosed in U.S. Pat. No. 6,001,749 to Child et al. This patent teaches a method in which areas of a fabric are coated with a repellant finish that inhibits the deposition of a conductive coating. The fabric is then coated with a conductive polymer leaving the pre-treated areas substantially free from the conductive polymer. This method leaves the highly colored conductor on the face and back of the fabric, thus detrimentally affecting the appearance, hand, and/or permeability of the fabric. Accordingly, both U.S. Pat. Nos. 6,001,749 and 5,624,736 are generally more suited to applications in Electromagnetic Interference Shielding (EMI).
Thus, a need exists for an economically manufactured fabric with relatively permanent anti-static properties that are inherent at virtually any humidity and does not affect the overall appearance or tactile properties of the fabric.